Batteries are the most common method to power portable systems. One popular type of battery is a lithium-ion battery. Such batteries have a wide voltage variation, from 4.2V when it is fully charged down to 2.7V when almost completely discharged. Therefore, in order to maintain high efficiency for power usage in a battery powered device, a switching DC-DC converter is used to convert the battery voltage level to the voltage level required by the circuit.
If the voltage required is always below or above the battery range than a buck or boost converter can easily carry-out this task, but in many applications the voltage required is in the middle of the range such that the voltage can not be supplied in either boost only mode or buck only mode. For this case a classic buck-boost converter design with 4 switches (two at the input for the buck and two at the output for the boost) that toggle all the time would be a well-known design choice. However, this configuration is very inefficient since the power consumed to drive the switches is at least twice that as when in boost only or buck only mode. The average current in the inductor would be much higher, when compared to a buck or boost only solution. Moreover, for the same reason the inductor required in the DC-DC converter would be larger, more expensive, and require higher saturation current
Numerous prior art systems have been proposed to improve the performance of buck-boost DC-DC converters. Mostly commonly such proposals have been focused on the three different modes (buck, buck-boost and boost) depending on the input/output voltage trying to minimize as much as possible the duration or amount of time spent in buck-boost mode, which is very inefficient. However, the mixed buck-boost mode usually cannot be avoided because there is a limitation on the maximum achievable duty cycle (typically 90%) making it impossible to transition directly from buck mode to boost mode. Moreover, this limitation introduces an additional problem because it is important to accurately match the two transition points for buck mode to buck-boost mode and from buck-boost mode to boost mode. Considering the process variation and inaccuracy during manufacture, it is difficult to establish this accuracy between transition points and such attempts can be very complicated and result in instability.
The only solution that can completely avoid the buck-boost mode was a current mode DC-DC converter, in this case it is possible to achieve good performances but at the cost of complex additional analog circuitry to accurately measure the current in the inductance. This complexity is a drawback to the prior art because it adds costs and may reduce reliability.
The following describe related prior art attempts at addressing the drawbacks of the prior art.
Current Feedback Buck-Boost Solutions without 4 Switches:
                1) U.S. Pat. No. 6,275,016 Buck-Boost Switching Regulator        2) U.S. Publication No. 2011/0156683 Current Mode Buck-Boost DC-DC Controller        3) U.S. Publication No. 2011/0187336 Non-Inverting Buck-Boost Voltage ConverterBuck-Boost Converter Solutions with 4 Switches:        4) The LTC3533—2A Wide Input Voltage Synchronous Buck-Boost DC/DC Converter        5) U.S. Pat. No. 7,737,668—Buck Boost Switching Regulator        6) U.S. Pat. No. 7,777,457—Constant Frequency Current-Mode Buck-Boost Converter With Reduced Current Sensing        7) U.S. Publication No. 2010/0045254—Average Current Mode Controlled Converter Having A Buck Mode, A Boost Mode And A Partial 4 Switch Mode        
To overcome the drawbacks in the prior art, an improved DC-DC controller is disclosed.